1. Field of the Invention
The present invention relates to a power steering system for vehicles, and more particularly, to an electric power steering system for vehicles which produces auxiliary steering torque by means of a steering servo device using an electric motor.
2. Description of Relevant Art
In recent years, in view of problems in hydraulic type power steering systems such as that the structure thereof is complicated, a variety of electric type power steering systems for vehicles have been proposed.
As one of such electric power steering systems for vehicles, there is disclosed an example of an analogue control type in UK Patent Application Publication No. GB 2132950 A, published July 18, 1984.
The electric power steering system for vehicles according to this UK Publication includes an input shaft as a steering shaft connected to a steering wheel, an output shaft interconnected at one end through a universal joint with the input shaft and at the other end through a rack and pinion type gear with a tie rod of steered wheels, an electric motor for supplying auxiliary torque through a reduction gear to the output shaft, a torque detecting mechanism disposed on the input shaft to detect steering torque acting on the input shaft, a motor drive circuit for driving the electric motor, and an analogue type control circuit for feeding the motor drive circuit with a control signal in accordance with a detection signal from the torque detecting mechanism.
The analogue type control circuit is adapted to have an armature voltage PWM-controlled to be impressed to the electric motor, with a polarity determined such that, during a steering operation in either direction of rotation of the steering wheel, the motor is urged to rotate in a direction corresponding to the steering direction. And, a signal corresponding to an armature current is fed back to the control circuit. Like this, the electric motor is adapted to produce controlled torque, which is supplied as the auxiliary torque through the reduction gear to the output shaft, whereby it is contrived to render the steering force light.
Incidentally, not limiting to electric power steering systems, the steering system in general has two different states imaginable with respect to the steering condition thereof, that is, a positive steering state and a negative steering state. The negative steering state can be regarded as a returning state of a steering wheel.
On the other hand, in a vehicle of which the steered wheels are front wheels as in most cases, when it wheels with the steered wheels turned in either direction while traveling, the front wheels have restoring forces acting thereon with tendencies to return the front wheels to their neutral positions. The restoring forces result from the wheel alignment of the front wheels and besides self-aligning torque due to deformations of tyres of the front wheels.
In this respect, if the steering wheel is manipulated with a steering force larger than required to overcome the restoring forces acting on the steered wheels, the steered wheels are caused to turn from the steering wheel side. Such state is the positive steering state. To the contrary, while the restoring forces acting on the steered wheels are so large as to overcome steering forces applied to the steering wheel, the steering wheel is caused to rotate from the steered wheel side. Namely, the steering wheel tends to be returned to its neutral position. Such state is the negative steering state, which corresponds to a returning state of the steering wheel and shall hereinafter be called "steering wheel returning state". The steering wheel returning state appears at such an occasion that, while a vehicle is wheeling in either direction, the driver tries to return the steering wheel to its neutral position, taking the steering wheel or leaving off.
In electric power steering systems with an input shaft of steering operation, in general, in the positive steering state the direction of steering torque acting on the input shaft is coincident with the direction of rotation of the input shaft, and in the steering wheel returning state the former is opposite to the latter.
In the meanwhile, not limiting to the case of the power steering system according to the UK Publication, in various electric power steering systems proposed in recent years, their steering servo devices include friction elements such as an electric motor and a reduction gear. In such servo device, however, an armature voltage to be impressed to the electric motor in accordance with steering torque is determined as a function taking into consideration a load from the road surface side only.
Therefore, in the case of the electric power steering system according to the UK Publication for example, at such a phase of the positive steering state that a manipulation of the steering wheel from the neutral position to left or right is started at a low speed with a relatively small steering force, the armature voltage becomes small. Thus, there appears a steering torque region in which no auxiliary torque that corresponds to such a load on the steering operation as due to the friction elements of the system is produced. In such torque region, it is needed to rotate the friction elements, that is, the electric motor and the like, from the side of the steering wheel. Accordingly, when starting a manipulation of the steering wheel, the steering operation may feel heavier than the case of a manual type steering system, so that the steering feeling may be degraded.
In this concern, it is generally known that between armature voltage Va and armature current Ia of an electric motor, there is such a relation that Va=Ia.multidot.Ra+K.multidot.Nm, where Ra is the internal resistance of the motor, Nm is the speed of rotation of the motor, and K is a constant in terms of induced electromotive force of the motor. Moreover, in the UK Publication above, the rotation speed Nm of the electric motor is proportional to the steering speed of the steering wheel. In this respect, however, even if the armature voltage Va to be impressed to the electric motor were controlled in consideration of an induced voltage Vi (Vi=K.multidot.Nm) of the motor, such problems as described could have 10 not been overcome. In other words, even if the induced voltage Vi of the electric motor, which Vi is proportional to the steering speed of the steering wheel, were taken into account in addition to the road surface side load to control the armature voltage Va, the above problems could have not been overcome. This is because a duty value attributable to the induced voltage term K.multidot.Nm of the armature voltage Va becomes small when the manipulation from the neutral position of the steering wheel is started at a low speed.
Further, in the steering wheel returning state, the returning characteristic of the steering wheel is to receive effects from some of the friction elements, in particular of the electric motor. More particularly, in the state in concern, since the steering wheel is caused to reversely rotate from the steered wheel side, also the electric motor is urged from the same side through the reduction gear to rotate in an opposite direction to the acting direction of steering torque. Incidentally, for the electric motor is adapted to rotate at relatively high speeds to output auxiliary torque through the reduction gear to the steered wheel side, the gear ratio of the reduction gear generally is set considerably larger than 1. In this respect, in that state in which the motor is urged to rotate from the steered wheel side through the reduction gear, such gear ratio functions in the form of a reciprocal number. In other words, the gear ratio of the reduction gear in the steering wheel returning state is reciprocal to that in the positive steering state. As a result, the returning characteristic of the steering wheel receives a corresponding effect, so that the steering feeling may be degraded.
With such points in mind, the present invention has been achieved to effectively solve such problems in conventional electric power steering systems as described.